Remove 3.0, 3.1, and 2.6 to shrink the test matrix.

Oh, and kill off ruby-head.
Prepare a 2.5.0 release.
Merge pull request #243 from apenney/2.4.2-release

2.5.0 release.
Goodbye, create_resources.
Merge pull request #237 from apenney/travis

Remove 3.0, 3.1, and 2.6 to shrink the test matrix.